Paraffin Fuel and Additive Combustion in an Opposed Flow Burner ...

AIAA 2012-3963

48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 30 July - 01 August 2012, Atlanta, Georgia

Paraffin Fuel and Additive Combustion in an Opposed Flow Burner Configuration Christopher R. Zaseck1 and Steven C. Shark1 Purdue University, School of Aeronautics and Astronautics, West Lafayette, IN, 47906 Steven F. Son2 Purdue University, School of Mechanical Engineering, West Lafayette, IN, 47906

Downloaded by Purdue Univ Lib TSA on December 31, 2012 | http://arc.aiaa.org | DOI: 10.2514/6.2012-3963

and Timothée L. Pourpoint3 Purdue University, School of Aeronautics and Astronautics, West Lafayette, IN, 47906

This paper describes the evaluation of paraffin fuel additives with an opposed flow burner. An opposed flow burner can provide regression rate data by consuming solid fuel samples with an impinging gaseous oxidizer jet. Baseline experiments, conducted at oxygen volumetric flow rates ranging from 8 to 50 SLPM, show that hybrid motor and opposed burner results agree for the relative regression rates of HTPB and paraffin. With baseline measurements established, the relative regression rates for polymeric, metal, and energetic additives to paraffin were measured at 25 SLPM. The polymeric additives decrease the opposed burner regression rate as expected, but at a larger magnitude than seen in hybrid rocket literature. Nano and flake aluminum did not readily combust in the opposed burner, due to short residence times and the disparity between aluminum ignition and entrained molten paraffin temperatures. In addition, flake aluminum presumably increased melt layer viscosity and decreased opposed burner regression rates. Because ammonia borane has a decomposition temperature below the vaporization point of paraffin, it likely decomposes within the melt layer. However, the added energy from ammonia borane did not increase regression rates as expected due to slow decomposition. Mechanically activated titanium carbon and pyrophoric titanium chromium manganese increase the regression rate of paraffin by 47% and 31% respectively. It is believed that both additives reacted or decomposed in or near the melt layer, and provided heat to the fuel before entrainment. Though the opposed burner is a useful tool, the results from this study indicate that its applicability may be limited to certain additive types.

I. Introduction

T

HE nature of hybrid rockets makes small scale fuel characterization difficult without full scale motor testing. In contrast, researchers often use strand burns to compare and test solid propellants in lieu of motor tests 1. Recently, researchers from The Pennsylvania State University used an opposed flow burner to achieve small scale hybrid fuel testing akin to propellant strand burns2, 3. An opposed flow burner uses an impinging oxidizer jet to combust solid fuel. The apparatus can be used to determine relative regression rates in order to compare fuels. As such, it is a useful tool for rapid screening of fuels and small scale combustion experiments. Young et al. obtained particularly interesting results using the opposed burner to examine the effect of magnetic fields on regression rate and to examine inverse hybrid oxidizers with a gaseous fuel 4, 5. The opposed burner could be useful to examine the combustion of new and novel paraffin fuel based fuels. In the last decade paraffin fuel technology has taken important steps to facilitate the widespread use of safe, high 1

Graduate Student, Aeronautics and Astronautics, 500 Allison Rd. West Lafayette IN, Student Member. Associate Professor, Mechanical Engineering, 500 Allison Rd. West Lafayette IN, Senior Member. 3 Research Associate Professor, Aeronautics and Astronautics, 500 Allison Rd. West Lafayette IN, Senior Member. 1 American Institute of Aeronautics and Astronautics 2

Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


performing, and economical hybrid rockets. Paraffin’s main strength is its high regression rate caused by the entrainment of molten paraffin. This entrainment mechanism enhances the regression rate of paraffin up to four times that of traditional fuels and greatly alleviates some standard problems associated with hybrid rockets 6, 7. However, the fuel is a brittle fuel and only achieves ~0.35% elongation at break and has a tensile strength of 0.863 MPa8. In contrast, hydroxyl terminated polybutadiene (HTPB) based solid propellants have an elongation and tensile strength of ~36% and 5.3 MPa respectively8. Researchers have addressed the mechanical properties of paraffin through polymer and strengthener addition, but have abated regression rate in the process8-10. However, even the unhindered regression rate of paraffin is insufficient for high thrust hybrid rockets. Casilas et al. showed, in a theoretical study, that a 12 fold increase over the standard hybrid regression rate could facilitate a single ported hybrid grain design for a high thrust, 93 inch OD motor11. The regression rate penalties of strengthening and the demanding requirements of high thrust motors point to the need for further regression rate enhancement. Metal and energetic additives have been beneficial in hybrid rocket applications, and could potentially improve the regression rate, combustion efficiency, and theoretical specific impulse of paraffin fuels. In fact, motor tests show that additives such as aluminum, carbon black, and ammonia borane can increase the regression rate of paraffin12, 13. To test a battery of prospective additives takes a significant amount of time and cost. Potentially, the opposed burner could facilitate rapid screening of these additives, and accelerate material development. The goal of this paper is to examine the regression rate of enhanced paraffin fuels using an opposed flow burner. Baseline experiments were performed to establish the relationship between neat HTPB and neat paraffin. Polymeric additives were explored in order to examine and establish the validity of strengthener testing on the opposed burner apparatus. In addition, we explored additives with varying combustion regimes and heat transfer mechanisms. In particular, heat release in the melt layer, near the melt layer, and in the flame zone was examined through a variety of metal and energetic additives.

II. Experimental Methods A. Materials and Sample Preparation Various vendors provided the materials for this study. The paraffin wax used in this study was from Sigma Aldrich and has a solidification point between 50-52°C14. We used a hotplate to melt the paraffin pellets at 100°C, and subsequently hand-mixed all additives directly into the fuel. The resulting molten mixtures poured easily into half inch OD straws for casting. We rapidly cooled each sample with ice water to provide a consistent cooling mechanism and prevent additive settling. This study investigated 79% active Novacentrix 80 nm nano-aluminum (nAl) as an additive15. We included 2.5 wt.% of Sigma Aldrich Span™ 80 (sorbitan oleate) in all nAl mixtures to promote dispersion and diminish agglomeration14. The surfactant sorbitan oleate can suspend aluminum in n-decane, an alkane similar to paraffin 16. In this study, we tested 5 wt.% and 10 wt.% nAl in paraffin. We also examined German Blackhead (Skylighter Chemicals) and Betonal 405 (Poudres Hermillon) flake aluminum with 80 wt.% and 93 wt.% molecular aluminum respectively17, 18. The German Blackhead flake aluminum has a stearin coating, an average size of 3 µm, and nanoscale thickness. The Betonal 405 flake aluminum has an average size of 14.5 µm, and thickness of 0.1 µm, and a specific surface area of 4.63 m2/g19. We did not use surfactant with the flake aluminum because the flake morphology tends to suspend easier. The Betonal flake was tested at 5%, 10%, and 15% by weight, and German Blackhead was tested at 10 wt.%. Two polymeric additives were examined in this study. Specifically we used Marco Polo International Inc. ethylene vinyl acetate (EVA) 2896 and the Kraton® polymer D111320, 21. The EVA and D1113 polymers have a melt index of 6 g/10 min and 24 g/10 min respectively. In addition to being less viscous, the Kraton ® polymer is more elastic in nature and can achieve 1500% elongation at break while the EVA achieves 820% elongation. Because the polymers required higher temperatures and longer mix durations, we used a hot plate and stir bar overnight to mix the polymers at 140°C. We varied the polymers from 2.5% to 7.5% by weight in paraffin. Ammonia borane (AB) was also examined in this study. The AB was synthesized using the procedure described by Ramachandran et al. 22. A Spex 6850 Freezer Mill pulverized the small AB bricks into fine powder. The mill ran for three one minute cycles at 10 Hz with one minute breaks between cycles and a one minute pre-cool. We mixed the resulting powder with 70°C molten paraffin to avoid AB decomposition. Overall the powder did distribute evenly throughout paraffin samples, though the small particles into larger ones agglomerated at 5 wt.%. We examined activated titanium-chromium-manganese (TiCrMn) as an additive. The TiCrMn was from Great Western Technologies and was described by Voskuilen et al.23. Hydrogen storage activation increased the exposed un-oxidized surface area of the TiCrMn and made it pyrophoric. To prevent oxidation and reaction, the TiCrMn 2 American Institute of Aeronautics and Astronautics


was mixed with molten paraffin in a humidity controlled argon glove box, and the samples were frozen using chilled isopropyl alcohol. When exposed to air the TiCrMn/paraffin samples showed no sign of reaction. The mechanically activated titanium and carbon used in this study (TiC) contained a 1:1 ratio of titanium to carbon. The titanium and carbon were dry milled on a Retch Planetary Ball mill for 5 minutes at 650 rpm. The ball to powder ratio was 40:1 using 6 grams of powder in a 250 ml jar. The resulting TiC was air passivated for 5 minutes in a fume hood to reduce its sensitivity. This procedure produced particles with a ~5 microns diameter. We included 5 wt.% of TiC in paraffin for this study. Carbon nanotubes were examined to increase the thermal conductivity of paraffin. The multiwall carbon nanotubes used in this study are from Cheap Tubes Inc., and have an outer and inner diameter of ~15 nm and ~4 nm respectively24. The nanotubes are 10-30 µm long and 95% pure. Due to quantity limitations, we only tested 1.5 wt.% carbon nanotube in this study. B. Opposed Flow Burner Operation We used an opposed flow burner to determine the relative regression rates of the enhanced paraffin fuels. The opposed burner apparatus generates an oxygen jet directly towards a small fuel sample as shown in Figure 1. The nozzle is 7.5 mm in diameter and is positioned one centimeter above the fuel surface. Each ~11.5 mm OD sample is held in a 12 mm ID steel mount. A spring loaded Omega LD620-25 linear variable displacement transducer (LVDT) enters the mount at the base and supplies a constant force on the fuel directed towards the nozzle. A 30 gauge tungsten wire is fastened over the mount and fuel sample that resists the force of the LVDT spring. As a result, the top surface of the fuel sample is restricted and the bottom surface travels at the regression rate of the fuel. The LVDT tracks the bottom of the pellet, and therefore records the regression rate. The opposed burner apparatus can support extra instrumentation for videography, spectroscopy, or PLIF. For this study, we recorded the majority of burns using a standard speed Cannon XL2 3CCD Video Camcorder.

Figure 1. A) Opposed flow burner schematic. B) HTPB Combustion. The LVDT outputs a calibrated voltage to an oscilloscope that, we later convert to position data, shown in Figure 2. The position data is numerically differentiated via the technique presented by De Levie et al. to determine the instantaneous regression rate of a sample25. Note that Figure 2 demonstrates the regression of HTPB. Paraffin generally burns for ~5-7 seconds with steady state combustion that lasts for ~1.5 seconds.

3 American Institute of Aeronautics and Astronautics


Figure 2. Top) HTPB LVDT data. Oxygen is supplied via 2500 psi 99.5% pure oxygen cylinders and routed via copper tubing. An Omega FMAA2317 flow meter measures the flow rate of the gaseous oxygen. For this study, we varied the oxygen flow rate from 8 to 50 standard liters per minute (SLPM) to compare HTPB and paraffin, and compared all other additives at 25 SLPM. To accommodate changing nozzle standoff height, the flow rate data was normalized using a bulk strain rate estimation. Strain rate is frequently used in opposed flow literature and is defined as the change in strain in a fluid element. Strain rate gives an indication of how intense the deformation of a fluid element is, and in opposed burner applications the bulk strain rate is a function of oxidizer velocity and nozzle standoff height. We used the video recording of each burn to determine changes in nozzle standoff and calculate strain rate. The bulk strain rate, a, is calculated by

a

v fuel  voxidizer h

,

(3)

where vfuel is the fuel velocity, voxidizer is the oxygen velocity, and h is nozzle standoff height. In all calculations vfuel is very small, and can generally be neglected.

III. Results and Discussion A. Baseline Tests To establish links between motor and opposed burner results, and determine potential limitation of the opposed burner, we compare the results in this paper to literature where possible. As a baseline for this study we determined the regression characteristics of neat paraffin and HTPB over a range of 8-50 SLPM. The baseline trends (Figure 3) show that HTPB and paraffin hold a similar relation in hybrid motor and opposed burner tests. For both hybrid rocket and opposed burner experiments the regression rate of paraffin is generally four times greater than HTPB26. The opposed flow burner removes molten paraffin in a manner akin to entrainment, proving it viable to compare binders. However, it remains to be seen if additive combustion will also translate to the opposed flow burner.

Figure 3: Left) HTPB/Paraffin opposed burner data and corresponding trends. Right) HTPB/Paraffin hybrid motor trends. Paraffin trend from Karabeyloglu et al. 26 and HTPB trend from Sutton27. 4 American Institute of Aeronautics and Astronautics

B. Polymeric Additives We will focus on opposed flow combustion at an arbitrary flow rate of 25 SLPM for additive comparison. The opposed burner polymeric additive results are shown in Figure 4.

1.25 1 0.75 0.5

Figure 4 specifically shows that neat paraffin regresses ~5 times faster than HTPB at 25 SLPM, and includes error bars representing the standard deviation. This again is comparable to hybrid motor data for which paraffin regresses ~3-4 times faster than HTPB26. The inclusion of polymeric additives decreases the regression rate of paraffin during opposed burner testing, as expected from literature. For example Maruyama et al. saw a 35% hybrid rocket regression rate drop due to the increased melt layer viscosity with 20 wt.% EVA9. The opposed burner testing shows a similar trend, with 2.5 wt.% EVA decreasing the regression rate of paraffin 34%. Though opposed burner data shows the expected regression rate trends, it is possible that at low oxidizer velocity the regression is more sensitive to melt layer viscosity. In addition, the less viscous Kraton® polymer does have a consistently higher regression rate on the opposed burner.

C. Metal and Energetic Additives The metal and energetic additive results are shown in Figure 5 and Figure 6. Figure 6 shows the data normalized by strain rate, and includes the linear fit for neat paraffin as a function of strain rate. 2 1.5 1

Ne at

Ne at

0

HT PB Par aff in 5% nA 10% l nA 5% l Be t o nal 10% Be to n 1 al 10% 5% B e to n Ge al rm an Bla ck 5% AB 5% TiC 1.5 rM % n Ca 5 %T rbo i/C nN ano tub es

0.5

Figure 5. Metal and energetic additive regression rate bar graph. 5 American Institute of Aeronautics and Astronautics

ton %

Kra

on 7.5

Kr a t

Kra % 2.5

Figure 4. Polymeric additive regression rate bar graph – All additives were applied to paraffin only.

5%

ton

in raf f Pa Ne at

EV A 7.5 %

EV A 5%

on

EV A 2.5 %

on

Kr at 7.5 %

5%

Kr at

on Kr at 2.5 %

aff Par Ne at

HT PB Ne at

in

0.25 0

Regression Rate (mm/s)


Regression Rate (mm/s)

1.5

2.2


Regression Rate(mm/s)

2 1.8 1.6 1.4

Neat Paraffin Fit 5% TiC 5% TiCrMn 5% nAl 5% Betonal 10% Betonal 15% Betonal 10% German Blackhead 5% AB

1.2

1 0.9 400

500

600

700

800 900 1000

1200 1400 1600 1800

Strain Rate (s-1) Figure 6. Metal and energetic additive regression rate vs. strain rate. Metal and energetic additives have a more ambiguous effect on opposed burner regression rates. Nanoaluminum rarely ignited visibly in the opposed burner, and did not increase the regression rate. The ignition temperature of aluminum, ~2350 K for micron sized and as low as ~930 K for nano sized particles, is well above the vaporization temperature of paraffin, 727 K7, 28. In a hybrid motor, the aluminum likely entrains into the flame zone and then ignites. In this case the burning aluminum provides heat feedback to the grain and increases the regression rate of paraffin, as shown by Evans et al.12. In addition, droplets containing aluminum may burn faster, jet, and micro-explode similar to aluminized n-decane and JP-8 droplets16, 29. Microexplosions and jetting could decrease droplet residence times, increase mixing, and improve the combustion efficiency in a hybrid rocket. However, in the opposed burner configuration the majority of entrained aluminum is guided away from the flame location and does not ignite. The aluminum that does ignite is rapidly blown away from the fuel and provides minimal heat feedback to the regressing surface. Because aluminum combustion seems to play a minimal role in the opposed burner, regression rate trends may not hold between opposed burner and hybrid motor experiments for metals that ignite within the flame region. Entrainment was minimal and agglomeration was an issue in mixtures containing 10 wt.% nAl. With the 10 wt.% nAl opposed burner tests the aluminum did not entrain with the paraffin, and caked on the fuel surface (Figure 7). The aluminum extended up to 5.2 mm above the retaining tungsten wire, and combusted with the incoming gaseous oxygen. In addition, local erosion caused the random liberation of small aluminum agglomerates from the surface. As a result of agglomeration, periodic aluminum release, and inconsistent heating, the 10 wt.% nAl regression rate was very inconsistent. It is unclear if a similar mechanism occurs in hybrid rocket motors.

Figure 7. A) Paraffin + 10 wt.% nano-aluminum combustion. B) Paraffin + 10 wt.% nano-aluminum combustion products. 6 American Institute of Aeronautics and Astronautics


Betonal flake aluminum decreased the regression rate of paraffin in the opposed burner. The flake aluminum combustion is similar to nano aluminum in that particles do not ignite regularly during an opposed burner test. Flake aluminum, particularly Betonal, can increase the host fluid viscosity and even cause shear thickening 19. Because Betonal increases the melt layer viscosity and adds very little energy to the system, the regression rate of the paraffin/Betonal is slower than neat paraffin. The combustion of German Blackhead flake does seem to occur more frequently, and from past research we know that it does not increase viscosity or cause shear thickening like Betonal 40519. The lower viscosity and limited aluminum combustion of the German Blackhead flake has minimal or no effect on the regression rate of paraffin in an opposed burner. Ammonia borane (AB), formula H3NBH3, is an interesting additive because it decomposes exothermically 380 K below the boiling point of paraffin. Presumably, the AB decomposes within the melt layer or in an entrained droplet. The exothermic decomposition should increase blowing and heat release within the melt layer, which could potentially increase regression rates. In fact, Weismiller et al. did show a moderate increase in regression rate by including 10 wt.% and 20 wt.% AB in a paraffin hybrid motor13. In the opposed burner, AB decomposition and subsequent boron combustion yielded a green and visibly brighter flame than other cases (See Figure 8). However, 5 wt.% AB caused a distinct 50% drop in the regression rate of paraffin at 25 SLPM. The inconsistency between motor and opposed burner trends, and the scattered data points obtained by Lee et al. (Figure 9), point to the need for additional work with the additive2. Though the bright green flame suggests AB decomposition and combustion, a white residue was apparent on the surface of the paraffin after combustion had extinguished. The residue was likely a mixture of combustion products and un-combusted AB. It is possible that the entrainment forces in the opposed burner configuration are not sufficient to remove decomposing AB from the fuel surface. As a result, slow AB decomposition may be the limiting step in opposed burner fuel regression. Also the size of the bright flame suggests that heat feedback due to the AB flame was far and removed (estimated 45 mm from the centerline). In addition, the residence times of the AB particles that were entrained may not be long enough to provide energy to the fuel during combustion. In any case, AB did not provide significant energy to the fuel and as a result AB decomposition does not augment the opposed burner regression rate. The limited effect of AB addition on regression rate may be alleviated by reducing particle size, and improving particle dispersal during mixing.

Figure 8. A) Neat paraffin combustion. B) Paraffin + 5% ammonia borane combustion.

Figure 9. Ammonia borane opposed burner data from Lee et al. 2. 7 American Institute of Aeronautics and Astronautics


Titanium carbon and titanium chromium manganese had the most notable effect on the regression rate of paraffin. Mechanically activated TiC undergoes a highly exothermic solid phase reaction with a maximum 3200 K adiabatic temperature30. The ignition temperature of the mechanically activated TiC can be tailored using milling time. For our research the TiC ignites at temperatures below the boiling point of paraffin based on DSC analysis. The reacting TiC potentially provided a significant heat to the regressing paraffin at a position much closer than the flame zone. As a result, the regression rate increased 47% at 25 SLPM. However, heat release from the TiC reaction softened the paraffin. The tungsten wire penetrated into the fuel at startup, and shortened the steady state combustion duration. The nozzle standoff distance was reduced by ~2.3 mm and the strain rate increased from 940 s-1 to 1230 s-1. Though the strain rate increased, the regression rate is still higher than neat paraffin as shown in Figure 6. The opposed burner does seem to capture the effects of substantial heat release in or near the melt layer. Future research will be conducted with full scale static hybrid rocket motor experiments to confirm this result. One particularly interesting result of this study is the protection of TiCrMn from pyrophoric ignition by solidified paraffin. The protective nature of paraffin opens up a new class of additives that could increase combustion temperature, combustion efficiency, and regression rate. The pyrophoric TiCrMn increased the regression rate of paraffin 31% in the opposed burner configuration. The flame of TiCrMn only appear slightly brighter, as shown in Figure 10, but there must be some heat added due to pyrophoric combustion or chemical decomposition. Though TiCrMn may not be the ideal additive for rocket combustion, it does show that pyrophoric materials are usable and could be beneficial in rocket applications. Pyrophoric aluminum, metal hydrides, or other materials requiring protection could also be considered for future testing.

Figure 10. A) Neat paraffin combustion. B) Paraffin + 5% TiCrMn combustion. Carbon nano-tubes have the potential to dramatically increase the conductivity of paraffin, and thereby enhance regression rate. However, with only 1.5 wt.% carbon there was a significant agglomeration problem. The agglomerates reached heights of ~6 mm above the initial starting surface as shown in Figure 11. The agglomerates reacted similar to nAl, in that large agglomeration would periodically blow off and affect the regression rate. As a result the regression rate data were inconsistent for nano-tube regression. It is unclear whether the agglomeration would also occur inside a hybrid rocket motor fuel grain during operation.

Figure 11. Paraffin + 5% carbon nano tube combustion.



D. Opposed Burner limitations This study has shown that the opposed burner can be a useful and rapid method for regression rate testing, however the apparatus appears to have limitations for fuel additive comparison. The opposed burner is well suited at comparing neat fuels, as demonstrated by similar opposed burner and hybrid motor HTPB/paraffin relationships. Fuels with polymeric additives also follow trends from literature, though melt layer viscosity may impede entrainment more in the opposed burner configuration. In general, we believe the opposed burner is a practical tool to screen potential polymeric additives, strengtheners, and fuels. In particular the opposed burner could be useful to rank fuel and binder regression rates. In the case of strengtheners, the opposed burner could be used in combination with strength tests for fast evaluation. Further research is being conducted to relate the regression rate trends seen in the opposed flow apparatus to the regression rate trends seen in a rocket motor for direct comparison. In contrast to strengthener addition, entrainment can hinder metal and powder based additive combustion and can invalidate opposed burner results. The combustion problems were best demonstrated by the aluminum opposed burner tests. As stated previously, nano and flake aluminum did not combust readily in the opposed burner. Entrainment removes aluminum particles from the flame before ignition can occur. In addition, the oxidizer velocity is generally lower in an opposed burner than in a hybrid rocket. The entrainment forces are therefore smaller in the opposed burner, and agglomeration can be an issue. This is demonstrated by 10 wt.% nano aluminum results. Due to agglomeration and aluminum entrainment, the regression rate did not follow the expected trends. These results suggest that the opposed burner is not appropriate for testing and comparing paraffin additives which combust in the flame zone, or particles which readily agglomerate. Elevated pressure or larger sample size may help alleviate these issues. Ammonia borane also lowered the regression rate of paraffin, contrary to hybrid rocket motor tests. In theory, ammonia borane decomposes exothermically in the melt layer, and should provide heat to the propellant sample. However, the AB residue on the surface of combusted pellets suggests that the AB did not entirely entrain. If the AB was not entraining, its decomposition could be the limiting step in regression. It seems that the opposed burner apparatus is not adept at predicting the regression trends of even some decomposing paraffin additives. The highly exothermic reaction of TiC and the pyrophoric metal TiCrMn both increased the regression rate of paraffin in the opposed burner. The TiC presumably ignited in or near the paraffin melt layer, and the inter-metallic reaction provided heat feedback very close to the fuel surface. The pyrophoric reaction of TiCrMn also must ignite easier than aluminum, and provide enough heat to enhance opposed burner regression rates. Both inter-metallic and pyrophoric reactions have not been tested in hybrid motors with paraffin, so the importance of the trends is unknown. However, both reactions have the potential to provide heat to the fuel at a position closer than the diffusion flame. We will examine this in future hybrid rocket motor experiments.

IV. Conclusion The main objectives of this work were to evaluate paraffin fuel additives using an opposed flow burner and to identify effective additives for larger scale testing. We showed that opposed burner and hybrid motor tests agree through baseline testing of HTPB and paraffin. The polymeric additive EVA also followed literature results by decreasing the regression rate of paraffin. However, as an additive to paraffin, 2.5 wt.% EVA in the opposed burner and 10 wt.% EVA in a hybrid motors caused an equal regression rate decrease. As expected, the less viscous Kraton® D113 polymer showed higher regression rates than EVA. The opposed burner could likely be used in conjunction with mechanical stress tests for rapid polymer screening. The results for metal and energetic additive combustion were less clear. Flake and nano aluminum did not easily combust in the opposed burner, and was likely expelled before it could ignite. In addition, the nano aluminum agglomerated heavily and skewed the data at 10 wt.%. Because aluminum combustion plays a minimal role, we believe that the opposed burner apparatus may not predict hybrid motor regression trends for metal additives that combust in the flame zone. The regression rate trend of AB and paraffin samples differed between opposed burner and motor test results. Though ammonia borane decomposes at melt layer temperatures, the solid decomposition products did not entrain easily into the flow. Ammonia borane decomposition was likely the limiting step for regression in the opposed burner. Presumably the AB is entrained in actual motors, and provides heat to increase regression rates. The important result is that some decomposing chemicals may not provide meaningful trends in the opposed burner configuration. Mechanically activated TiC increased the regression rate of paraffin ~47% in the opposed burner. The intermetallic reaction between the titanium and carbon is highly exothermic and can occur in the paraffin melt layer without oxygen. The added heat in close proximity to the fuel surface caused the regression rate to increase. This may indicate heat release in the melt layer as a particularly effective method to increase the regression rate of 9 American Institute of Aeronautics and Astronautics

paraffin. Paraffin also protected the TiCrMn from pyrophoric ignition thereby showing the possible use of a new, and energetic, class of materials for regression rate and performance enhancement. In addition, the TiCrMn increased the regression rate of paraffin by 31%.

Acknowledgments The work presented here is supported under NASA’s Space Technology Research Fellowship provided to the first author. In addition, the authors would like to thank George Fletcher, George Story, and Greg Zilliac from NASA for their advice and guidance.

References


1

Hinkelman, M. C., Heister, S. D., and Austin Jr, B. L. "Evaluation of Solid Composite Propellants Using a Dicyclopentadiene Binder," 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit. San Diego, CA, 2011. 2 Lee, J., Weismiller, M., Connell, T., Risha, G., Yetter, R., Gilbert, P., and Son, S. "Ammonia borane based-propellants," 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Hartford, CT, 2008. 3 Young, G., Risha, G. A., Miller, A. G., Glass, R. A., Connell, J., Terrence L., and Yetter, R. A. "Combustion of AlaneBased Solid Fuels," International Journal of Energetic Materials and Chemical Propulsion Vol. 9, No. 3, 2010, pp. 249-266. 4 Young, G., Koeck, J. J., Conlin, N. T., Sell, J. L., and Risha, G. A. "Influence of an Electric Field on the Burning Behavior of Solid Fuels and Propellants," Propellants, Explosives, Pyrotechnics Vol. 37, No. 1, 2012, pp. 122-130. 5 Young, G., Roberts, C., and Dunham, S. "Combustion Behavior of Solid Oxidizer/Gaseous Fuel Diffusion Flames," 50th AIAA Aerospace Sciences Meeting Nashville, Tennesse, 2012. 6 Karabeyoglu, M., Cantwell, B., and Altman, D. "Development and Testing of Parraffin-based Hybrid Rocket Fuels," 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Salt Lake City, UT, 2001. 7 Karabeyoglu, M. A., Altman, D., and Cantwell, B. J. "Combustion of liquefying hybrid propellants: Part 1, general theory," Journal of Propulsion and Power Vol. 18, No. 3, 2002, pp. 610-620. 8 DeSain, J. D., Brady, B. B., Metzler, K. M., Curtiss, T. J., and Thomas, V. "Tensile Tests of Paraffin Wax for Hybrid Rocket Fuel Grains," 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. August ed., Denver, CO, 2009. 9 Maruyama, S., Ishiguro, T., Shinohara, K., and Nakagawa, I. "Study on Mechanical Characteristic of Paraffin-Based Fuel," 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. August ed., San Diego, CA, 2011. 10 Galfetti, L., Merotto, L., Boiocchi, M., Maggi, F., and De Luca, L. T. "Ballistic and rheological characterization of paraffinbased fuels for hybrid rocket propulsion," 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. August ed., San Diego, CA, 2011. 11 Casillas, E. D., Shaeffer, C. W., and Trowbridge, J. C. "Cost and Performance Payoffs Inherent in Increased Fuel Regression Rates," 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 33rd. Seattle, WA, 1997. 12 Evans, B., Boye, E., Kuo, K., Risha, G., and Chiaverini, M. "Hybrid Rocket Investigation at Penn State University's High Pressire Cumbustion Labaratory: Overview and Recent Results," 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Denver, CO, 2009. 13 Weismiller, M. R., Connell Jr, T. L., Grant, A. R., and Yetter, R. A. "Characterization of Ammonia Borane (NH3BH3) Enhancement to a Paraffin Fueled Hybrid Rocket System," 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Nashville, TN, 2010. 14 Sigma Aldrich: Chemical Distibutor, url: http://www.sigmaaldrich.com [cited July 2012] 15 Novacentrix: 80nm Aluminum Distributor, url: http://www.novacentrix.com [cited July 2012] 16 Gan, Y., and Qiao, L. "Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles," Combustion and Flame Vol. 158, No. 2, 2011, pp. 354-368. 17 Skylighter Chemicals: Aluminum Distributor, url: http://www.skylighter.com [cited July 2012] 18 Poudres Hermillon: Aluminum Distributor, url: http://www.poudres-hermillon.com/ [cited July 2012] 19 Sippel, T. R., Zaseck, C. R., Wood, T. D., Pourpoint, T. L., and Son, S. F. "Combustion of aluminum and ice propellants: New formulations for improved processing, performance, and cost," Proceedings of the Central States Section of the Combustion Institute 2010 Technical Meeting. Champaign, IL, 2010. 20 Marco Polo Internation Inc.: Polymer and Resin Distributor, url: http://www.mar-pol.com/ [cited July 2012] 21 Kraton Performance Polymers, Inc.: Polymer Distributor, url: www.kraton.com [cited July 2012] 22 Ramachandran, P. V., and Gagare, P. D. "Preparation of Ammonia Borane in High Yield and Purity, Methanolysis, and Regeneration," Inorganic Chemistry Vol. 46, No. 19, 2007, pp. 7810-7817. 23 Voskuilen, T., Zheng, Y., and Pourpoint, T. "Development of a Sievert apparatus for characterization of high pressure hydrogen sorption materials," International Journal of Hydrogen Energy Vol. 35, No. 19, 2010, pp. 10387-10395. 24 Cheap Tubes Inc.: Carbon Nanotubes Supplier, url: www.cheaptubes.com [cited July 2012] 25 De Levie, R., Sarangapani, S., and Czekaj, P. "Numerical Differentiation by Fourier Transformation as Applied to Electrochemical Interfacial Tension Data," Analytical Chemistry Vol. 50, No. 1, 1978, pp. 110-115. 26 Karabeyoglu, A., Zilliac, G., Cantwell, B. J., DeZilwa, S., and Castellucci, P. "Scale-up tests of high regression rate paraffin-based hybrid rocket fuels," Journal of Propulsion and Power Vol. 20, No. 6, 2004, pp. 1037-1045. 27 Sutton, G. P., and Biblarz, O. Rocket Propulsion Elements: John Wiley and Sons, 2001.



28 Yetter, R. A., Risha, G. A., and Son, S. F. "Metal Particle Combustion and Nanotechnology," Proceedings of the Combustion Institute Vol. 32, No. 2, 2009, pp. 1819-1838. 29 Hedman, T., Cho, K., Pfeil, M., Satija, A., Mongia, H., Groven, L., Son, S., and Lucht, R. "High Speed PLIF Applied to Multiphase Combustion," Spring Technical Meeting of the Central States Section of the Combustion Institute. Dayton, OH, 2012. 30 Varma, A., and Lebrat, J.-P. "Combustion synthesis of advanced materials," Chemical Engineering Science Vol. 47, No. 9– 11, 1992, pp. 2179-2194.
